Conventional hard disk drives (hereinbelow, referred to as HDDs) rotate magnetic disks and drive head gimbal assemblies (hereinbelow, referred to as HGAs) at high speed in response to requests for huge capacity, high recording density, and high-speed accessing. These cause substantial fluctuation of air (turbulence) that buffets the magnetic disks and HGAs. This turbulence buffeting can lead to disturbance in positioning heads for data which are recorded on a magnetic disk with high-density. This is because the turbulence occurs at random and it is difficult to estimate its magnitude and cycle so that swift and accurate positioning control becomes complex and difficult. The turbulence buffeting may also cause noise to impair the quietness of the device.
Another issue caused by the influence of the air within the device due to the high-speed rotation, is an increase of electric power consumption. When the magnetic disk is rotated at high speed, the air around the disk is dragged and rotated together. On the other hand, the air away from the magnetic disk remains still so that shearing force arises therebetween to become a load against the rotation of the disk. This is called as windage loss, which becomes larger as the disk rotates at higher speed. In order to rotate the disk at high speed against the windage loss, a motor will require larger output and more electric power.
Given that the above-described turbulence and windage loss are proportional to the density of the gas within the device, there is an idea to reduce the turbulence and windage loss by enclosing low-density gas instead of air in a hermetically-sealed HDD. Hydrogen, helium, or the like is exemplified as the gas having a lower density than air, but helium is optimum because it is effective, stable, and safe in considering actual use. HDDs with sealed helium gas can solve the above-described issues and realize swift and accurate positioning control, power saving, and satisfactory quietness.
However, molecules of helium are so small and the diffusion coefficient of helium is large. Therefore, there has been an issue that enclosures used for usual HDDs are poorly sealed so that helium gas leaks easily during normal use. In order to make it possible to hermetically seal low-density gas such as helium gas, for example, a technique disclosed in a U.S. Patent Publication No. 2005/0068666 (“Patent Document 1”) described below has been suggested.
Since helium gas escapes easily as described above, welding or solder jointing is considered as appropriate for sealing an HDD. If an HDD is determined to be defective upon inspection after assembly, parts in the HDD are replaced to repair the HDD. In order to perform this repair easily, it is desirable not to weld or solder joint until the inspection is completed after helium gas has been enclosed.
The above-described Patent Document 1 discloses a method for sealing the HDD with doubled covers as an approach to solve this dilemma. This method seals the HDD, until the end of the inspection step, with a primary cover having a low helium-gas-permeable gasket which can be easily disassembled and replaced. Then, after passing the inspection, it attaches a secondary cover and seals the HDD by welding or soldering the second cover. Thus, utilizing a double cover structure enables the HDD to be hermetically sealed with helium gas and the components to be disassembled and replaced easily after the inspection.
In the step of fixing the secondary cover by welding or soldering, dust or gas may be generated. Especially, generation of gas in welding will be an issue. If the dust or gas enters the interior space where a magnetic disk and the other components are placed, reliability in a head-disk interface will be reduced. Therefore, it is important to prevent the dust or gas from entering the interior space in joining.
On this occasion, in order to prevent the helium gas from leading from the finished HDD, the welded or soldered sealing section of the secondary cover should be inspected and confirmed that the helium gas is not leaking. However, if the above-described double cover structure is adopted, since a certain level of sealing property is secured by the primary cover, leaks may not be able to be detected at the inspection step notwithstanding the presence of a leak hole at the sealing section of the secondary cover.
Then, in order to prevent intrusion of dust or gas into the interior space in joining or to assure the leak inspection on the secondary cover, a ventilation hole may be provided on the primary cover to let a certain quantity of helium gas flow out therethrough while joining the secondary cover. However, if the ventilation hole is merely provided on the primary cover, large amounts of helium gas may leak at the time of fixing the secondary cover so that the necessary helium gas may not be left in the HDD. On the other hand, if the flow-out amount of the helium gas is too little, it may be impossible to securely prevent intrusion of dust or gas, or to conduct a reliable gas leak inspection.